A method and an apparatus for controlling the braking system of a vehicle are known, for example, from German Patent No. 41 12 845. In the compressed-air braking system described there, the application pressure of the individual wheel brakes, in the event of a brake actuation, is determined by analyzing the change in braking pressures over time. The application pressure that is ascertained is used to correct the reference pressure, derived from the driver's inputs, for the individual wheel brakes so as to produce a braking force of equal magnitude at all the vehicle wheels or at the vehicle wheels of one axle. The corresponding action is also used upon release of the brake to ascertain the release pressure. It has been found, in this context, that when the hystereses of the wheel brakes during pressure buildup and pressure reduction are different, the application and release pressures of the individual wheel brakes or of the brakes of the subvehicles of a vehicle combination may lie far apart from one another. This leads to different braking forces at the individual wheel brakes or within the individual subvehicles. This leads to nonuniform wear on the wheel brakes and to longitudinal forces between the subvehicles. It is particularly problematic to ascertain the application and/or release pressures in the case of trailers or semitrailers for which no information about the current status of the wheel brakes is available.
In the case of the known apparatus, the pressure values are sensed upon application of the brakes. Alternatively, it is possible to sense values which represent the control variables (e.g. corresponding control signals) to be defined for application or release of the brakes. The latter is true, in particular, for braking systems without a fluid (gaseous) braking medium. The discussion hereinafter will therefore, as a generalization, refer to application and release control variables, the preferred exemplary embodiment referring to hydraulic or pneumatic braking systems.